In general, it is known that nitrogen included in metals such as titanium alloys and steel has various influences on the properties of the metals. Specifically, nitrogen is an interstitial solid-solution strengthening element and a chemical element which is effective for increasing strength by stabilizing the a phase of a titanium alloy and steel. On the other hand, it is known that nitrogen decreases the toughness of steel.
Therefore, for example, in the steel industry, the chemical composition of the products is adjusted on the basis of a nitrogen analysis value in the middle of a refining process in order to manufacture steel products having desired properties. Therefore, there is a demand for a method for rapidly determining nitrogen content in a metal sample with a high accuracy, as a method for analyzing nitrogen in metal, which can be used for adjusting a chemical composition in the middle of a refining process.
Conventionally known examples of a method for analyzing nitrogen in steel include wet methods such as a distillation-bispyrazolone spectrophotometric method and an inert gas fusion thermal conductivity detection method (Non Patent Literature 1). In particular, as a method for analyzing a nitrogen component in a steel-manufacturing process, an inert gas fusion thermal conductivity detection method is widely used from the viewpoint of analysis rapidness.
In addition, in the case of metals other than steel, it is known that wet analysis methods and an inert gas fusion thermal conductivity detection method are used (Non Patent Literatures 2 through 4).
An inert gas fusion thermal conductivity detection method is a method in which nitrogen content in a metal sample is determined by melting a metal sample in a graphite crucible in an inert gas stream (carrier gas) in an impulse furnace, by flowing nitrogen gas generated by the fusion of the sample into a thermal conductivity detector, and by determining the thermal conductivity of the carrier gas containing nitrogen gas.
In the case where an inert gas fusion thermal conductivity detection method is used, the following problems exist.
A gas generated from a metal sample is not limited to just nitrogen. For example, in the case of a steel sample, it is known that, when a steel sample is melted in an inert gas atmosphere in a graphite crucible, hydrogen gas and carbon monoxide gas are generated in addition to nitrogen gas, as disclosed in Patent Literature 1. These gases are generated as a result of hydrogen and oxygen contained in the steel sample gasifying or reacting with the graphite crucible to form gases. Such gases coexisting with nitrogen (hereinafter, referred to as “coexisting gases”) decrease the accuracy of nitrogen content determination. In particular, carbon monoxide gas, which has a thermal conductivity close to that of nitrogen gas, causes a large analysis error in the case where it is not removed. Therefore, when nitrogen in a steel sample is analyzed, it is necessary to remove coexisting gases before transporting generated nitrogen gas to a thermal conductivity detector in order to determine the content of nitrogen gas. In order to remove coexisting gases, first, by passing gases generated from a melted steel sample through a column filled with oxidation catalyst, carbon monoxide gas and hydrogen gas are respectively oxidized into carbon dioxide and water vapor. Subsequently, generally, carbon dioxide gas is removed by passing the oxidized gases through a CO2 remover composed mainly of sodium hydroxide, and then, water vapor is removed by passing the gases through a dehydrator composed mainly of magnesium perchlorate. By flowing the mixed gas of nitrogen gas and a carrier gas from which coexisting gases, which have a negative effect on analysis, have been removed to a thermal conductivity detector in order to determine thermal conductivity, nitrogen concentration in the steel sample is calculated from the relationship between the detected thermal conductivity and nitrogen concentration and the weight of the sample determined in advance. Here, since there is a decrease in the capability of reagents used in the oxidation column, the CO2 remover, and the dehydrator described above due to determination being repeatedly performed on samples, the reagents are periodically replaced in order to maintain satisfactory analysis accuracy. However, in the case where a steel sample is analyzed in a practical steel-making process, there is a case of an abnormal nitrogen analysis value even though the removal of the coexisting gases and the replacement of the reagents are rigorously practiced.
In addition, in the case where nitrogen analysis is performed by using an inert gas fusion thermal conductivity detection method, there is a problem in that it is necessary to use expensive helium gas as a carrier gas. In a thermal conductivity detection method, since the amount of change in thermal conductivity of gases is a signal value, the sensitivity of a detector increases with increasing difference in thermal conductivity between a carrier gas and an analysis target gas. Conversely, in the case of an analysis target gas having a thermal conductivity close to that of a carrier gas, since a change in thermal conductivity is small, detection is difficult. That is, in the case where nitrogen gas, which has a comparatively low thermal conductivity, is detected, helium, which has a high thermal conductivity, is the only carrier gas option available. Also, Non Patent Literatures 1 through 4 mentioned above state that helium should be used.
However, since helium is a gas whose abundance ratio in the air is very small, unlike argon, which is another kind of inert gas, producing helium by separating it from the air is not economically viable, and helium is produced by refining crude helium gas which is produced along with a natural gas. Therefore, helium gas is produced only in some particular countries, and there is a case where its supply is stopped depending on the political situation of the producing countries. In addition, nowadays, since there is a significantly growing demand for helium gas for use as a coolant or for medical purposes, the price of helium gas is very high, and a further price increase is anticipated. Therefore, an inert gas fusion thermal conductivity detection method, which requires the use of helium, is a method which incurs high cost and which has difficulty persisting.
Moreover, examples of a method for analyzing nitrogen in steel include spark atomic emission spectrometry, which is described in Non Patent Literature 5, Patent Literature 2, and Patent Literature 3. This method is a method in which, by inducing spark discharge on the surface of a steel sample for several seconds, and by determining light emitted when nitrogen atoms in an excited state generated from the surface of the sample return to the ground state, nitrogen concentration in the sample is derived, and this method is known to be excellent in terms of rapidness. However, there is a problem in that spark atomic emission spectrometry is poor in terms of the accuracy or precision of analysis values. This is, for example, because it is not easy to excite nitrogen so as to emit light, since nitrogen has a higher ionization energy than other chemical elements. In addition, since the analytical wavelength of nitrogen atoms is 149 nm, which is within the vacuum ultraviolet region, it is not easy to stably detect the light due to, for example, absorption by oxygen, significant attenuation by optical systems such as mirrors and lenses, and long-period attenuation caused by the degradation of the surfaces of optical systems over time. Although Patent Literature 2 and Patent Literature 3 mentioned above disclose techniques for solving such problems, since the absolute quantity of the steel sample to be determined is so small as to be less than 1 mg in the case of spark atomic emission spectrometry, the representativeness of the data is low, which results in fundamental limitations on the improvement of accuracy.
Here, nitrogen concentration in molten steel is analyzed by performing nitrogen analysis and adjusted in secondary refining processes including one in which a decarburization furnace is used and subsequent processes in which, for example, a RH vacuum degasser is used. Patent Literatures 4 through 6 discloses techniques in which target nitrogen concentration is achieved by analyzing nitrogen concentration in molten steel immediately before nitrogen-concentration adjustment is performed and by controlling a nitrogen-concentration-adjusting treatment on the basis of the analytical result. Usually, nitrogen concentration in molten steel is determined by charging a sample taken from the molten steel into an inert gas fusion thermal conductivity detection apparatus. However, as described above, there is a potential problem of obtaining an abnormal nitrogen analysis value with an inert gas fusion thermal conductivity detection method. For example, as Patent Literature 6 indicates, in the case where argon gas is circulated in an RH degasser immediately before an analysis sample is taken, since there is a case where the taken sample contains argon gas, a nitrogen analysis value higher than the practical value is derived if the portion containing argon gas is analyzed by using an inert gas fusion thermal conductivity detection method. It is needless to say that, in the case where a nitrogen analysis value is incorrect as described above, nitrogen concentration of finally obtained molten steel is different from a target nitrogen concentration.